THE ODORIFEROUS VOLATILE CHEMICALS BEHIND THE OXIDATIVE AROMA DEGRADATION OF SPANISH RED WINES

Abstract

It is a well-established fact that premature oxidation is noxious for wine aromatic quality and longevity. Although some oxidation-related aroma molecules have been previously identified, there are not works carrying out systematic research about the changes in the profiles of odour-active volatiles during wine oxidation.

Different types of wines in terms of region, grape variety, oak aging and price were subjected to an oxidative aging procedure, sensory analysis, gas-chromatography olfactometry (GC-O) and quantitative analysis. Sensory notes such as dried fruit, cooked vegetables or liquorice-alcohol were oxidation-related. The GCO analysis of the samples with highest oxidation notes, revealed highest levels of four odour zones, which were identified in a dual system GC-O/FID-GC-O/MS as 1,1-diethoxyethane (liquor, strawberry, sweet), 2,4,5-trimethyl-1,3-dioxolane (fruity, solvent), 3-methylbutanal (solvent, yeasty) and methional (boiled potato, cooked vegetables).

The two aldehydes were quantified by gas chromatography-mass spectrometry (GC-MS). together with isobutanal, 2-methylbutanal and phenylacetaldehyde. All them were already present in significant amounts before oxidation. However, as they were forming odourless reversible adducts with SO₂ (α-hydroxyalkylsulphonates)1 they were initially non-odour active. However, as free SO₂ disappeared during oxidation² they become odour-active in oxidized samples. Additional quantities were formed during oxidation, most likely by the reaction of wine dicarbonyls with the amino acid precursors. This additional formation was particularly relevant for 2-methylbutanal, followed by methional and isobutanal, while for phenylacetaldehyde and 3-methylbutanal, quantities formed were smaller than those originally present. These results confirm that both, pre-existent levels of Strecker aldehydes and the ability to form them during oxidation, are relevant in wine stability.

Acetals were determined by L-L microextraction followed by GC-MS. Results revealed that during oxidation there is a clear increment on the levels of acetals formed from the condensation of acetaldehyde with ethanol, 2,3-butanediol and glycerol; leading to 1,1-diethoxyethane, 2,4,5-trimethyl-1,3-dioxolane and several heterocyclic acetals, respectively. Levels formed were high enough to be odour-active. This suggests that the formation of acetals is an essential part of the sensory changes noted during wine oxidation.